The present invention relates to an air-fuel ratio control system for an engine for a motor vehicle, and more particularly to a system which appropriately controls additional air for a caruberator of the engine with a feedback control system.
Recently, an air-fuel ratio control system for an engine provided with a feedback control system has been proposed. In such a system, an exhaust gas sensor such as an O.sub.2 sensor is provided for sensing the oxygen concentration of the exhaust gases to generate an electrical signal which is used for controlling the air fuel ratio of the fuel and air mixture.
FIGS. 5 to 9 show a conventional air-fuel ratio control system disclosed in Japanese Patent Application Laid-Open No. 53-82927.
As shown in FIG. 5, an O.sub.2 sensor 2 provided in an exhaust passage 1 of the engine detects the oxygen concentration of the exhaust gases and produces an electrical signal which is applied to a buffer amplifier 3 for amplifying the signal. The amplified signal is applied to a peak-to-peak voltage providing circuit 5 (hereafter called P-P circuit) and an air fuel ratio control circuit 4. The P-P circuit 5 produces upper and lower peak voltages in the output of the amplifier 3, the output signals of which are applied to a reference value circuit 6. The circuit 6 produces a mean value of the peak voltages as a reference value for a desired air-fuel ratio of the air-fuel mixture. The output signal corresponding to the reference value is applied to the air fuel ratio control circuit 4 and compared with the output signal of the amplifier 3. The output signal of the control circuit 4 is supplied to an actuator driving circuit 7 for operating an actuator 8. The actuator 8 operates to actuate an air bleed control valve in a carburetor (not shown) for controlling the flow rate of intake air or to control the amount of fuel injected from a fuel injector.
In the system, since the reference value is determined based on the peak values of concentration of oxygen in the exhaust gases, the reference value does not change even if the output characteristic of the O.sub.2 sensor 2 changes because of its deterioration with time. However, since the O.sub.2 sensor 2 has a high internal resistance and is located near the engine, noise such as ignition noise from an ignition system is liable to affect the output of the O.sub.2 sensor.
FIG. 6 shows an example of an electric circuit for the system of FIG. 5 and FIG. 7 shows waveforms showing characteristics of output signals of the circuit.
The O.sub.2 sensor 2 produces an output signal VO.sub.2 including alternate maximum peak values and minimum peak values in accordance with the variation of the oxygen concentration. If an abnormal high voltage signal Vnoise enters into the O.sub.2 sensor 2, the O.sub.2 sensor 2 produces a signal having a high voltage which is charged in a capacitor C1 of the P--P circuit 5 as a peak value Vpeak. High voltage at capacitor C1 continues until the higher peak voltage is discharged. In accordance with the higher peak voltage, the reference value circuit 6 produces a reference value Vs1 which is higher than a predetermined reference value Vs. The high reference value Vs1 is applied to an inverting input terminal of an operational amplifier OP1 of the air fuel ratio control circuit 4 and compared with the signal VO.sub.2 applied to a non-inverting input terminal thereof. Accordingly, the amplifier OP1 produces an output signal which is greatly deviated from an ordinary value. The deviated signal is further applied to a non-inverting input terminal of a comparator OP2 and compared with a triangular wave pulse train from an oscillator 12 to produce a square wave pulse train. The square wave pulse train operates to turn on-off a transistor Tr. Thus, the actuator 8 is intermittently operated at an abnormal duty ratio. Accordingly, an improper amount of intake air is supplied, thereby reducing exhaust emission control.
As shown in FIG. 8, the Japanese patent application further discloses a system in which the reference value circuit 6 has a minimum value limiter 16 comprising a diode D1, and resistors R1, R2, and a maximum value limiter 17 comprising a diode D2, and resistors R3, R4. When the reference value exceeds a predetermined maximum value or a predetermined minimum value, either of the diodes D1, D2 is forward-biased to limit the reference value to the maximum value or the minimum value.
As shown in FIG. 9, when a higher peak voltage Vpeak is applied to the reference value circuit 6, the maximum value limiter 17 operates to limit the peak value to the maximum value VsLimit which is higher than the predetermined reference value Vs. The maximum value VsLimit is used as the reference value. However, the maximum value continues until the higher peak voltage charged in the capacitor C1 is discharged and gradually approaches the reference value as shown by a line Vs1. Accordingly, afore-mentioned defects cannot be removed by the system.